Specify what the load is, along with the required motor current and voltage. In the US, you'll get an effective 150-300 W/m^2 for incoming power; this has to be multiplied by the efficiency of the solar cells. If you figure a solar cell is roughly 1/4 efficient, then you're looking at being able to get 40-75 W/m^2 of effective power. Clearly, to answer your question, one would have to know the power consumption of the motor under load. Otherwise, your question is like "How long is a piece of rope?"